Studies of the transforming proteins of the small DNA tumor viruses, such as Adenovirus E1A, have provided a wealth of information about the proteins and pathways that regulate cellular proliferation. Though interactions with key cellular transcription factors, such as the Retinoblastoma tumor suppressor, E1A reprograms cellular gene expression to inhibit differentiation and promote cell cycle progression. Research in my lab has led to the identification and characterization of a novel cellular protein, TIR, that antagonizes the transcriptional activation and cellular transformation activities of the Adenovirus E1A oncoprotein. TIR specifically represses expression of certain cellular and viral promoters dependent on cell cycle control elements recognized by E2F. The studies described in this proposal are designed to test the hypothesis the, through interaction with the Retinoblastoma tumor suppressor and related proteins, TIR acts to down-regulated the activity of specific transcription factors such as E2F that are important for cell cycle progression and thereby contributes to the loss of proliferation characteristics of terminal differentiation. The specific aims of this research are to (1) investigate TIR function in proliferation and differentiation in cell culture assays, (2) identify functional domains of TIR and correlated binding to specific proteins with the promoter specific repression and cellular effects mediated by TIR, and (3) investigate the mechanism by which TIR represses transcription assays. In addition to advancing out knowledge about the specific function of TIR, these studies are likely to provide important information about the regulation of E2F- dependent transcription, the roles of pRb and related proteins during differentiation, and perhaps provide novel insight into the mechanisms of transcriptional repression. Studies of TIR may also yield unique information about the cellular pathways targeted by DNA tumor viruses in viral replication and oncogenic transformation. Since disturbances of the finely tuned transcription program often result in developmental abnormalities or predisposition to diseases such as cancer, studies of regulators such as TIR are imperative in order to increase our understanding of these processes at the cellular and molecular level.